Abstract: Target course and speed are important features of the marine target. Accurately acquiring target course and speed have important significance for target tracking and recognition. When the radar carrier has a high speed, the radar measurement of target position is nonlinear.The sea targets is slow .In a short period, limited by the accuracy of radar, the small changes of target position is difficult to precisely estimate. It need to built a high precision mathematical model, using Kalman filtering method to improving the estimation accuracy of target position. For monopulse radar, this paper presents a strong real-time calculating method of offshore target speed and course. The nonlinear Kalman filter model is derived under the condition of high speed radar carrier. Using radar measurement and the target carrier INS information calculate the speed and course of marine target. The theoretical analysis of the factors of data length affecting the calculating accuracy is proposed, and the empirical formula of data length selection is given based on different target type. Finally, the simulation of the maritime targets in different scenarios verify the correctness of the calculation method.